Field of the Invention
The present invention relates to processing of image data in an image formation of an electrophotographic method.
Description of the Related Art
As exposure methods employed in an exposure unit of an electrophotographic image forming apparatus, there are an LED exposure method and a laser exposure method. The LED exposure method arranges a plurality of LED elements that are light-emitting elements in a lengthwise direction of a photoreceptor, and provides a plurality of lenses that focus light outputted by the LED elements on the photoreceptor. The laser exposure method has a light source unit that emits a laser beam by a semiconductor laser that is a light-emitting element, and a scanning unit that performs a laser beam deflecting scan by a polygon mirror. The laser exposure method further guides the laser beam from the light source unit to the scanning unit and has a plurality of lenses for forming an image using the laser beam, with which a deflecting scan is performed by the scanning unit, on the photoreceptor.
It is desirable for a light intensity distribution formed on a photoreceptor surface (hereinafter, a spot shape) to be approximately circular, and it is desirable for the size of the spot shape (hereinafter, spot diameter) to be approximately uniform irrespective of a position on the photoreceptor surface. Therefore, light output from the light-emitting element is designed so as to form an image by approximately uniform spot diameters on a photoreceptor surface after passing through a lens group.
In recent years, there are design examples in which, for an objective of miniaturization or a cost reduction, lens characteristics are simplified and spot diameters are not necessarily uniform. In addition, even with a design in which spot diameters are made to be uniform, there are cases in which there is an effect from distortion due to assembly error or a manufacturing error of a component part or a supporting body, so spot diameters change, and uniform spot diameters cannot be achieved. Nonuniformity of spot diameters appears in an output image as a difference in a tone characteristic depending on the scanning position, and causes so-called inplane uneven density to occur.
Japanese Patent Laid-Open No. 2006-349851 (hereinafter, PTL 1) discloses a technique for holding, with respect to each position in a main scanning direction, a plurality of two-dimensional tables for performing density correction in accordance with tonal values of an input image. To allow sufficient suppression of inplane uneven density by this technique, it is necessary to increase the number of the two-dimensional tables to be held for the density correction. By PTL 1, a test pattern having uniform density in a main scanning direction and a density gradient in a sub scanning direction is formed, a density of the test pattern is detected, and a correction table for correcting density unevenness of the main scanning direction is created. The test pattern is something that arranges a plurality of patches at equal intervals on an entire region of the main scanning direction.
By the technique of PTL 1, although an optimal correction table can be obtained for representative points that divide the main scanning direction into equal intervals (16 points in accordance with FIGS. 4 and 8 of PTL 1), correction residuals occur at other points. To have sufficiently small correction residuals, it is necessary to increase a number of divisions of the main scanning direction. However, increasing the number of divisions leads to an increase of a number of correction tables.